1. Field of the Invention
This invention relates to the art of magnetic recording and more particularly, to a magnetic recording medium using ferromagnetic fine powder which is improved in electromagnetic conversion characteristics and durability. The invention is particularly suitable for a disk-shaped magnetic recording medium.
2. Description of the Prior Art
Magnetic recording mediums such as audio tapes, tapes for video tape recorders, magnetic sheets, coating-type magnetic disks and the like have a yearly tendency toward a higher recording density and a shorter wavelength in recording signals. With coating-type magnetic recording mediums, ferromagnetic powder dispersed in the magnetic recording layer becomes finer in size and higher in packing density, thereby improving electromagnetic conversion characteristics as is known in the art. This eventually leads to a very smooth surface of the magnetic recording medium. The smoothness results in an increase in contact area between the medium and a magnetic head with an increasing friction coefficient. Thus, the medium is more liable to be degraded than known mediums with respect to the runnability and durability and the wear resistance of the magnetic layer.
In order to improve the film strength of the magnetic recording medium and to obtain an appropriate effect of cleaning the magnetic head, there has been hitherto proposed the use of non-magnetic inorganic powders with a relatively high hardness in the magnetic layer such as, for example, .alpha.--Al.sub.2 O.sub.3, Cr.sub.2 O.sub.3, .alpha.--Fe.sub.2 O.sub.3, TiO.sub.2 and the like, with or without use of solid or liquid lubricants in combination. The use of such inorganic powders as an abrasive material is described, for example, in Japanese Patent Publication Nos. 57-49967 and 57-49969 and Japanese Kokai Nos. 60-5421, 60-7614 and 61-73240.
Efforts have been made for the high degree of packing or filling of the magnetic powder in the magnetic layer so as to meet the demand for the high density recording. This inevitably requires a reduction in amount of the abrasive, making it difficult to properly balance the electromagnetic conversion characteristics and the durability.
On the other hand, there has been proposed, for example, in Japanese Kokai No. 1-260626, a magnetic recording medium which makes use of an abrasive having an arborescent or chain-like structure in order to make a great surface roughness in small amounts. For the surface roughness which is so suppressed that little or no influence is produced on the electromagnetic conversion characteristics, the amount of the abrasive has to be reduced with an undesirable lowering of the film strength. Thus, satisfactory durability cannot be expected.